Conductive carbon particles are commonly used as fillers to enhance the conductivity in polymers, ceramics, coatings, and electrodes of electrochemical systems. For example, carbon conductive additives are used in a variety of primary and secondary batteries like alkaline zinc/manganese dioxide batteries, zinc carbon batteries, lithium primary and rechargeable batteries, nickel cadmium batteries, lead acid batteries, and nickel metal hydride batteries, lithium sulfur batteries, lithium air batteries, metal air batteries with metals like zinc or iron, fuel cells as well as capacitor systems.
Conductive additives are applied in electrodes of electrochemical cells to decrease the electrical electrode resistance. Carbonaceous powdered materials are often selected as conductive additives due to their light weight and inertness towards acidic and alkaline electrolytes. Conductive additives do not contribute to the electrochemical processes of the electrode, which means that for a high energy density of the cell, the applied quantity of conductive additive is desirably minimized. Typical carbon conductive additives used are fine graphite powders and conductive carbon black (see for example, M. E. Spahr, Lithium-ion Batteries-Science and Technology, M. Yoshio, R. J. Brodd, A. Kozawa (Eds.), Springer, New York, 2009, Chapter 5).
The addition of a small amount of conductive carbon to the negative electrode of a lead acid battery leads to an improvement of the cycle life and charge acceptance when the battery works in high-rate partial state-of-charge (HRPSoC) mode as for example applied in the use of hybrid electric vehicles (see for example, K. Nakamura, M. Shiomi, K. Takahashi, M. Tsubota, Journal of Power Sources 59 (1996) 153, M. Shiomi, T. Funato, K. Nakamura, K. Takahashi, M. Tsubota, Journal of Power Sources, 64 (1997), 147 and D. Pavlov, P. Nikolov, T. Rogachev Journal of Power Sources 196 (2011) 5155-5167). When a lead acid battery is operated at partial state-of-charge (PSoC) the irreversible formation of lead acid sulfate (“sulfation effect”) causes a significant reduction of the battery cycle life (see, for example, D. Pavlov, Lead-Acid Batteries-Science and Technology, Elsevier 2011, Chapter 1, pp. 23-26).
Besides using carbon additives, it is known in the art to use modified grid designs, glass fiber mats inside the active material, and/or modified electrolyte compositions as other ways to improve the conventional starting, lighting, ignition (SLI) lead acid batteries and make them useful for operation modes at lower states of charge (SOC) (cf., for example, D. Pavlov, Lead-Acid Batteries-Science and Technology, Elsevier 2011, Chapter 7). The battery characteristics obtained in these advanced lead acid batteries at shallow high rate discharge operations make them good candidates for micro- and mild hybrid electric vehicles.
The addition of graphite, expanded graphite, activated carbon, and carbon black to the negative electrode has been shown to result in an improvement of the cycle life of the lead acid batteries, primarily by a reduction of the sulfation effect.
Several hypotheses have been proposed to explain the mechanism of the carbon effect in the negative electrode. A survey of the influence of a wide spectrum of carbons has been summarized in the literature (P. T. Moseley, Journal of Power Sources 191 (2010) 134-138 and D. P. Boden, D. V. Loosemore, M. A. Spence, T. D. Wojcinski, Journal of Power Sources, 195 (2010) 4470-4493). It has been shown recently that the carbon should have a high affinity to lead in order to enable the formation of a carbon-lead skeleton in the negative electrode while plating lead during the electrode formation performed in the first charging of the fresh newly assembled cell (D. Pavlov, P. Nikolov, T. Rogachev Journal of Power Sources 196 (2011) 5155-5167). This carbon-lead skeleton increases the surface area and in addition the carbon provides an additional supercapacitor effect in the electrode, both of which provide possible explanations for the increased charge acceptance.
In addition to the electrical conductivity properties, conductive additives also have an effect on the electrode structure and porosity. For example, the electrolyte penetration of the electrode can be influenced by the electrode structure and porosity, which has an impact on the ionic resistivity of the electrode (see for example, M. E. Spahr, Lithium-ion Batteries-Science and Technology, M. Yoshio, R. J. Brodd, A. Kozawa (Eds.), Springer, New York, 2009, Chapter 5).
The positive electrode of a lithium sulfur battery contains sulfur mixed with binder materials and one or more carbon components. The carbon provides the electrical conductivity and in addition is thought to assure the dimensional stability of the electrode during the discharge of the cell when the sulfur content of the positive electrode is decreased by the formation of the discharge products (see, for example, Xiong, Shizhao; Hong, Xiaobin; Xie, Kai; Rong, Lixia, Huagong Jinzhan (2011), 30(5), 991-996 and Yao, Zhen-Dong; Wei, Wei; Wang, Jiu-Lin; Yang, Jun; Nuli, Yan-Na, Wuli Huaxue Xuebao (2011), 27(5), 1005-1016).
Furthermore, electrochemical cells with air electrodes, contained in fuel cell stacks or metal air batteries, can require carbons in the positive air electrodes. It is thought that the carbons act as support for the metal or metal oxide catalyst and also generate the structure providing dimensional stability to the electrode. In order to be used in air electrodes, carbon supports are required to demonstrate a high corrosion resistance to air or oxygen, as failure to do so is thought to limit cell durability (see for example, S. Sarangapani, P. Lessner, L. Swette, J. Giner, Proceedings—Electrochemical Society (1992), 92-11 (Proc. Workshop Struct. Eff. Electrocatal. Oxygen Electrochem., 1992), 510-22, S. Muller, F. Holzer, H. Arai, O. Haas, Journal of New Materials for Electrochemical Systems (1999), 2(4), 227-232 and F. Maillard, P. Simonov, E. Savinova, Carbon Materials for Catalysis (2009), 429-480).
As mentioned above, natural or synthetic graphite, expanded graphite, activated carbon and carbon black have all been used as conductive additives.
Graphite is crystalline carbon. The electronic conductivity of graphite is based on the crystal graphite structure which consists of stacked layers of six-membered carbon rings with delocalized electrons in conjugated p-orbitals parallel to the graphite layers. The electronic conductivity parallel to the stacked planes is about three orders of magnitude higher than the electronic conductivity perpendicular to the planes. This results in the known anisotropic behaviour of the electronic conductivity (A. W. Hull, Phys. Rev. 10 (1917) 661 and W. Primak, L. H. Fuchs, Phys. Rev. 95(1) (1954) 22).
The application of graphite as, for example, conductive additives could be attributed to properties such as its high compaction ability, which results in improvements in the electrode density of the cell. It has also been demonstrated that a carbon conductive additive can significantly increase the cycling stability and low temperature charge/discharge performance of the electrode. However, although the resistivity at high concentrations of graphite is very low, it has been observed that due to the higher percolation threshold for graphite compared to carbon black, relatively large amounts of graphite are required to decrease resistivity of the electrode.
High surface area graphite is typically obtained by decreasing the particle size of graphite in a milling process. To avoid the oxidation of the graphite product during milling, milling can be carried out in an inert gas atmosphere (see for example, N. J. Welham, J. S. Williams, Carbon 36(9) (1998) 1309-1315, T. S. Ong, H. Yang, Carbon, 38 (2000) 2077-2085 and Y. Kuga, M. Shirahige, Y. Ohira, K. Ando, Carbon 40 (2002), 695-701). A drawback of conventional milling processes is that activated carbon and high surface area graphite can contain a relatively high amount of trace metals due to the use of metal based milling equipment. Metal trace elements may act as electrocatalysts interfering with the desired electrochemical process and cause parasitic chemical or electrochemical side reactions which decrease the cycling stability and reduce the cell life.
Carbon black is an amorphous form of carbon. The carbon black structure is made up of typically spherical amorphous primary particles which are bound together by covalent bonds to form larger aggregates. Conductive carbon black typically consists of primary particles of 10-50 nm in size and large complex aggregates are often more than 100 nm in diameter. The conductive carbon black aggregates form a conductive network in porous electrodes thus decreasing the electronic resistance (J. B. Donnet, R. P. Bansal, M. J. Wang, in Carbon Black Science and Technology, 2nd ed., Marcel Dekker Inc., New York, 1993). The large intra- and inter-aggregate void volume of conductive carbon black created by the carbon black structure results in high oil absorption numbers. Conductive carbon blacks typically have oil absorption numbers above 150 mL/100 g (measured according to ASTM D2414-01, see method described below).
Another class of carbonaceous material is activated carbon. Activated carbon is composed of amorphous high surface area carbon powders derived from natural organic products like coconut shells or wooden products or polymers. These precursors are carbonized at temperatures between 700 and 1600° C. Subsequent to carbonization, the material is subjected to an activation process using steam, CO2, or aqueous zinc chloride solutions at elevated temperatures which increases the BET surface area of the carbonized material. The activation process forms so-called “micro-pores” which are thought to be the cause for the observed increase in surface area (see for example, H. Marsh, F. Rodriguez-Reinoso, Activated Carbon, Elsevier, 2006).
The use of carbon black as, for example, a conductive additive can be attributed to properties such as high liquid absorption, which appears to lead to a higher electrolyte penetration. Furthermore, the addition of the high surface area carbon component has been observed to result in a noticeable increase of the charge acceptance due to the increased electrochemically available inner electrode area, which appears to be a consequence of the more “open” structure of the electrode. A further explanation for the positive effect of carbon black additives is that the charging of the additional carbon surface (supercapacitor effect) may lead to an increased electrochemical capacity, which is a desired property in, for example, lead acid battery negative electrodes and supercapacitors.
However, despite the applications of high surface area carbons as carbon additives, some adverse consequences with respect to cycle life, performance at high rate and low temperature discharge have been observed. A further problem associated with high surface area carbon components is a high water up-take as a paste formulation, which may interfere with the production of the electrodes containing such additives.
Furthermore, the decomposition of the aqueous electrolyte, which happens as a parasitic side reaction in the lead acid battery during charging, leads to hydrogen formation at the negative electrode. It has been found that the electrolyte decomposition rate is accelerated by the high surface area of the carbon and in presence of typical metal impurities. Also, the oxygen formed in this reaction at the positive electrode could be a cause of oxidative carbon corrosion which seems to occur particularly with high surface area amorphous carbons.
It can be seen from the aforementioned properties that conductive carbon additives appear to have a significant impact on the electrode engineering, its properties, and the manufacturing process of the electrode.
As described above, graphite and conductive carbon black appear to have many complementary properties, when considering their use as conductive additives in electrodes. As both low and high surface area carbons (graphite and amorphous carbon powders) have shown to exert positive effects yet suffer from different drawbacks in the intended applications, attempts to use a mixture of the two have been described in the literature (see for example, M. Fernandez, Batteries & Energy Storage (BEST) Spring 2011 81-93 and M. Fernandez, N, Munoz, R. Nuno, F. Trinidad, Proceedings of the 8th International Conference on Lead Acid Batteries, Extended Abstract #6, Lead Acid Battery Department of the Bulgarian Academy of Science, Sofia, Bulgaria, Jun. 7-10, 2011, p. 23-28). However, such mixtures are fraught with problems. For example, in the manufacturing process of the negative electrode, the required homogeneous mixing of two carbon components, one of which has a very low volume density in the lead oxide paste formulation, can be problematic.
Accordingly, it is an object of the invention to provide an alternative carbon material which can be reliably made, is easy to handle and has excellent physicochemical and electrochemical properties, especially when used as a conductive additive, as well as methods for its preparation.